From 35cbb5751ed7ae578c9a6194404a516c9f9d2ef7 Mon Sep 17 00:00:00 2001
From: Kate F <kate@elide.org>
Date: Sun, 20 Sep 2020 22:05:12 -0700
Subject: [PATCH] Rewrite `(?:|xyz)` and `(?:xyz|)` to `(?:xyz)?`.

---
 src/libre/ast_rewrite.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/src/libre/ast_rewrite.c b/src/libre/ast_rewrite.c
index 4b17b0594..88f62dcf0 100644
--- a/src/libre/ast_rewrite.c
+++ b/src/libre/ast_rewrite.c
@@ -317,6 +317,30 @@ rewrite_alt(struct ast_expr *n, enum re_flags flags)
 		return 1;
 	}
 
+	if (n->u.alt.count == 2) {
+		struct ast_expr *child = NULL;
+
+		if (n->u.alt.n[0]->type == AST_EXPR_EMPTY) {
+			ast_expr_free(n->u.alt.n[0]);
+			child = n->u.alt.n[1];
+		} else if (n->u.alt.n[1]->type == AST_EXPR_EMPTY) {
+			ast_expr_free(n->u.alt.n[1]);
+			child = n->u.alt.n[0];
+		}
+
+		if (child != NULL) {
+			free(n->u.alt.n);
+
+			/* we repurpose the same node */
+			n->type = AST_EXPR_REPEAT;
+			n->u.repeat.e = child;
+			n->u.repeat.min = 0;
+			n->u.repeat.max = 1;
+
+			return rewrite(n, flags);
+		}
+	}
+
 	return 1;
 
 empty: